Ink jet printing is a non contact printing method in which tiny droplets of ink are ejected from nozzles onto a substrate. Several methods of droplet ejection exist including piezo, acoustic and thermal. Each method delivers a pressure pulse to the ink jet printing ink which results in the controlled ejection of an ink droplet towards a substrate. In thermal ink jet printing a voltage is applied to a resistor which rapidly heats up and vaporises some of the liquid vehicle in the ink jet printing ink so forming a vapour bubble. The increased pressure and expansion caused by the formation of the bubble forces a small amount of the ink to be ejected from the printer nozzle. The voltage is turned off and the vapour bubble collapses. Replacement ink is then drawn into the ink jet printer head completing the cycle. In today's thermal ink jet printers the cycle of steps for droplet ejection generally take place at high frequencies often over 1,000 times a second. A typical thermal ink jet printing nozzle should desirably fire well in excess of a million times over its lifetime.
Thermal ink jet printers should desirable fire reliably and precisely over the full lifetime of the printer head. Unfortunately, it is often the case that the print quality tends to degrade over time. Examples of print defects which can sometimes be observed include: missing pixels, missing lines and ink droplets being ejected in uncontrolled directions or in uncontrolled quantities.
It is often speculated that the heat cycling of the resistors in thermal ink jet printers is at least partly responsible for a slow degradation of components within the ink that can eventually impair the correct operation of the ink jet printer.
Dye-based inks (where the colorant is soluble in the liquid vehicle) tend to perform relatively robustly in thermal ink jet printers. In contrast, pigment-based inks (where the colorant is present as a particulate dispersion in the liquid vehicle) tend to perform significantly less well. One particular problem is that of pigment dispersion colloidal stability. Any pigment particles which aggregate or flocculate can easily foul the thermal resistor, coat the nozzles or block the ink jet printer nozzles. Pigment-based inks tend to have better properties such as light, ozone fastness and optical density when compared to dye-based inks, thus pigment-based ink suitable for thermal ink jet printing are desirable.
Accordingly, there is a need for dispersions which can be used to prepare thermal ink jet printing inks which address, at least in part, one or more of the above problems.